1. Field of the Invention
This invention relates generally to the field of multiplexers and demultiplexers. More particularly this invention relates to a method and apparatus for multiplexing and demultiplexing at very high data rates. The invention is particularly useful at data rates greater than 100 MBPS.
2. Background of the Invention
This invention addresses the problem of providing a low cost simple and reliable method and apparatus for multiplexing and demultiplexing packetized data at high data rates.
Many different types of multiplexing techniques are known in the art and are used in various situations to combine multiple sources of data into a single data stream. Some typical examples are Synchronous Time Division Multiplexing (TDM), Statistical Time Division Multiplexing (STATDM) and Packet SWitched Multiplexing (PSWM). The terminology used in connection with Statistical Time Division Multiplexing has not yet been fully standardized. For purposes of this document, STATDM is as defined in conjunction with FIG. 7.8 of the text "Data Communications", by D. R. Doll, Wiley-Interscience Publications, 1978. When any of these techniques are applied to a wideband network they fall far short of providing acceptable performance due to high implementation cost or other drawbacks. For example, with conventional TDM each data source is allocated a predetermined slot of time in a frame of data. If the data source is not providing data at any given time, it's time slot is still reserved and no data is transmitted during the predetermined time interval reserved exclusively for it. Conventional TDM is therefore potentially wastefull of bandwidth when it is used to transmit bursty data or in fact any data which is not likely to be in the form of a continuous stream of data.
STATDM alleviates the problem of poor bandwidth utilization somewhat by allowing statistical allocation of bandwidth according to need. This is generally accomplished by preceding each segment of data by a header including a flag and a field of address information which allows the link level of the STATDM system to determine where each segment of data is to be routed by reading the address. The flag is used to mark the beginning of the segment. Although this technique is more bandwidth efficient, it is substantially more complex to implement due to the requirement of a flag decoder and an address decoder. Moreover, it is very expensive and technically difficult at this time to implement an address decoder at speeds adequate to utilize STATDM above several hundred MBPS.
Packet switching is also difficult to implement at high frequencies for the same reasons as stated above in connection with STATDM. In a PSWM system, data packets are normally preceded by a header including a source address used at each level of the system, a destination address and possibly other information such as packet length. In order to properly route the data packet, this header must be read and interpreted much as in the case of STATDM. Therefore, although PSWM is generally considered to be among the most bandwidth efficient multiplexing techniques, it too is difficult to implement at high data rates. It should also be noted that both STATDM and PSWM are inherently asynchronous and generally require the use of buffering and resynchronization equipment in order to transmit and receive synchronous data.
In general, packet switched networks are capable of higher bandwidth utilization than corresponding circuit switch networks. It is therefore desirable in many instances to replace circuit switched networks with packet switched networks in order to achieve better bandwidth utilization. At very high data rates (for example greater than 100 MBPS) however, it is very difficult or expensive, as previously described, to process data packets due to the very short time available to route the packets. Although the routing problem can be alleviated somewhat by utilization of specialized hardware circuits to perform the packet switching, it is more desirable to utilize microprocessor technology to perform the packet switching so that flexibility of the system is retained. In a microprocessor system, the system can be individually tailored to varying situations by changing the microprocessor's programming.
The present invention provides a method of dealing with these problems by introducing a new type of multiplexing which will be referred to as STOchastic Time Division Multiplexing or STOTDM. This multiplexing technique can be combined with circuit switching and packet switching systems in order to achieve robust packet switching performance at data rates exceeding 100 MBPS. By utilizing standard Emitter-Coupled Logic (ECL) technology, the present invention can be operated as high as 200 MBPS at very low cost. With high speed ECL, speeds approaching 300 MBPS should be achievable. It is anticipated that utilization of gallium arsenide devices and other emerging technologies will permit the present invention to operate reliably at data rates greater than 2 GBPS in many applications.